feat: Update posthog-flutter to support wasm-builds

[flowhorn]

This is a potential PR for WASM-Support in posthog-flutter.

:bulb: Motivation and Context

Enables wasm-builds in web as it uses dart:js_interop instead of dart:js. Also the new functions give a little bit more type safety.

Closes https://github.com/PostHog/posthog-flutter/issues/112

:green_heart: How did you test it?

:pencil: Checklist

• [ ] I reviewed the submitted code.
• [ ] I added tests to verify the changes.
• [ ] I updated the docs if needed.
• [ ] No breaking change or entry added to the changelog.